<p>Managing requirements modeling and prototyping is risky. If things go awry, projects can spiral out of control. Over the years, with the help of colleagues from industry and academia, the author has identified a requirements modeling and prototyping process that is fast, powerful, cost-effective, sane, and objective. The main lesson learnt is that throwaway prototyping (sometimes called exploratory prototyping) is always cost-effective and always improves specifications. The process has nine steps: elicit initial requirements; model requirements; identify constraints; prioritize initial requirements; design; evaluate designs; specification; interactive prototyping; and requirements validation.</p>